The present invention relates generally to data communications, and more particularly to an improved method and apparatus for detecting errors in data communications, such as, for example, data communications in cellular radio telephone systems.
Radio telephone systems provide telephone service to handheld portables and vehicular mobiles by means of radio base stations, microcellular base stations, and control terminals coupled to telephone company phone system (TELCO) by telephone landlines. Such systems may also include cordless base stations located in a home or building and coupled to a telephone landlines for providing local telephone service when in the home or building (see, for example, U.S. Pat. No. 4,989,230). In such radio telephone systems, telephone calls are typically originated and terminated by means of data communications over dedicated signalling radio channel. Such data communications consist of one or more frames containing a synchronization word and a data word or message transmitted individually or in multiword packets. The synchronization word is selected to be a unique sequence of bits for defining the beginning of the following data word. In US cellular telephone systems, the data communications on the signalling radio channels is Manchester encoded and transmitted at a 10 kilo-bit rate. The format of the synchronization word and data words for US cellular telephone systems is specified in Electronic Industries Associates Standard EIA-553.
Since radio signals are subject to interference and fading, bit errors may be introduced during transmission and reception of the data. In order to detect the presence of bit errors, parity bits are typically included with each frame of data. If the parity bits indicate that a received data word has an error, the data word may be discarded. The parity bits may also be coded to allow correction of one or more erroneous bits using any of a number of coding schemes, such as, for example, a Bose-Chaudhuri-Hocquenghem coding scheme. Then, if one of more bits of the received data word are erroneous, the erroneous bits may be corrected using the parity bits. However, it is possible that correcting a received data word may transform it into a valid data word different from that which was transmitted. Bit errors may also result in false synchronization and processing of a meaningless data word.
Problems introduced by bit errors may be reduced by increasing the number of bits in the synchronization word, increasing the number of parity bits, or correcting fewer bit errors. However, increasing the number of bits in a frame decreases data throughput, and correcting fewer bits causes more data words to be discarded. Accordingly, there is a need for an improved method and apparatus for detecting and correcting errors in radio data communications for cellular radio telephone systems.